Reach the top, and everything will be yours. At the top of the tower exists everything in this world, and all of it can be yours. You can become a god. This is the story of the beginning and the end of Rachel, the girl who climbed the tower so she could see the stars, and Bam, the boy who needed nothing but her.
I liked it during the first episodes, but towards the middle I started having doubts and during the last episodes I hated it.
My final personal opinion is that the anime is quite arrogant: it really tries to be kind of epic, but its characters are too shallow (or just don't match) compared to the complicated story (and I have doubts as well about the story itself - e.g. a lot of missing informations, too many sudden changes of directions, ...).
Potentially still worth to be watched, but be careful with your expectations.